Increasing awareness of the effects of vehicle evaporative and exhaust emissions has resulted in regulations at both state and federal levels to control these emissions. In particular, on-board diagnostic regulations (e.g., OBDII) require that certain emission related systems on the vehicle be monitored, and that a vehicle operator be notified if the system is not functioning in a predetermined manner.
One example of an emission related system is a fuel system, which includes a fuel tank for storing fuel. Vapors from the fuel collect within the fuel tank. Occasionally, the fuel tank may develop a leak due to a hole, such as from a sharp object puncturing the fuel tank. Additionally, other components of the fuel system may develop leaks or otherwise begin to operate in a faulty manner. As a result, vapors present within the fuel system may inadvertently escape into the atmosphere. A primary component of the fuel vapor is hydrocarbon, which is known to have a detrimental effect on air quality. Currently, on-board diagnostic regulations require that a diagnostic small leak test and a very small leak test be performed periodically while the vehicle is operational, to detect a leak. As to the latter test, this diagnostic requires detection of leaks equivalent to an orifice of 0.50 mm diameter (0.020″) to be detected. If a leak is detected by the diagnostic test, the vehicle operator is notified. For example, on-board diagnostics may be configured to perform a leak detection test on the fuel tank as seen by reference to U.S. Pat. No. 6,311,548 entitled “METHOD OF VALIDATING A DIAGNOSTIC LEAK DETECTION TEST FOR A FUEL TANK”, issued to Breidenbach et al., assigned to the common assignee of the present invention. Breidenbach et al. disclose a fuel tank leak test in which a predetermined initial vacuum level is established in the fuel tank, and then the vacuum decay rate is monitored. A fuel tank leak would bleed the vacuum fairly quickly, failing the test. In the specific context of the fuel tank leak test, Breidenbach et al., also disclose that fuel slosh may affect the actual vacuum decay rate positively or negatively.
Additionally, it is known to run a diagnostic leak detection test on a purge control solenoid valve (PCSV). For this diagnostic, however, a decay in a vacuum level is not monitored. Rather, the purge control solenoid valve, which is coupled to the downstream or vacuum side of the throttle, is first closed. The diagnostic also calls for the closure of the vent valve, which as known is typically installed on the fresh air inlet of a charcoal canister. As further background, the vent valve, when open (normal operation), allows ambient air to enter the canister for use in replacing the purged vapor with the engine running. Further, when the engine is not running, as vapors are produced within the fuel system, they are collected by the charcoal canister, and then any remaining pressure is released through the vent. When the vent valve is closed (diagnostic operation), the vent allows the evaporative system to be closed off from the environment when the purge valve is additionally closed.
Then, as to the purge valve leak test, the fuel tank vacuum (pressure level) is monitored over time. If the vacuum increases beyond acceptance criteria, then the purge valve may be leaking. As to the purge valve leak detection test, one conventional leak detection approach may result in false failures. That is, this conventional diagnostic would indicate a failure of the purge flow leak test; however, subsequent testing shows the “failed” purge valve to be within leak specifications. This situation of falsely indicating that the purge leak test failed is undesirable, for example, resulting in increased cost (e.g., warranty claims) to inspect the system.
There is therefore a need for a method of evaluating the integrity of a purge valve leak detection test that minimizes or eliminates one or more of the problems set forth above.